The present invention relates to the use of structural cables in construction works such as bridges. In particular, the invention is applicable to suspension bridges and cable-stayed bridges.
In a suspension bridge, the deck is supported via hangers attached to one or more main suspension cables. Each suspension cable is anchored at both ends and deviated on one or more pylons erected along the bridge span. In a cable-stayed bridge, the deck is supported by a set of cables, called stays, each extending between a pylon and an anchorage mounted on the deck.
In most suspension bridges, the main suspension cables usually consist of a bundle of parallel metallic wires arranged side by side in a compact configuration. It has also been proposed to build the main suspension cables from seven-wire strands, each strand having six peripheral wires twisted around a central wire (see e.g. EP-A-0 950 762). Such strand is advantageously surrounded by a plastic sheathing which may further contain an anti-corrosion product such as grease or wax. That sort of strand is more frequently used in pre-stressing applications or to form stays in a cable-stayed construction (see e.g. EP-A-0 323 285).
The traction forces to which the cable is subjected are taken up by its metallic wires. For a given load capacity of the cable, the use of seven-wire strands leads to a cable having an overall cross-section significantly larger than a cable consisting of a compact bundle of parallel wires. Geometrically, the twisting of the wires in a strand requires more space than the compact stacking of parallel wires. In addition, the individual sheathing of the strands also occupies a certain space.
When the cable must include a large number of metallic wires, such as in large suspension bridges where a main cable typically has several thousands of wires, parallel wires are generally preferred to avoid having a too large cross-section of the cable. It is also an established technology.
In a cable-stayed arrangement, the load is distributed between a larger number of stays each having a smaller number of wires (typically between 100 and 1,000 wires), which makes it more practical to use prefabricated strands. However, it is sometimes required to minimize the diameter of the stays, in particular for aerodynamic reasons. Therefore, parallel wire cables are sometimes used in cable-stayed works as well.
However, a shortcoming of parallel wire cables is the bulk of their anchorage systems. Usually, the main cables on major suspension bridges are fabricated in situ from many steel wires laid out on a catwalk along the cable line and anchored by looping around a series of semi-circular cables shoes attached to an anchor block. Each shoe typically receives more than a hundred wires. At the anchorage, the cable shoes are distributed over a large surface and are themselves anchored in a massive structure. In addition, the fan distribution of the cable wires at the anchorage requires a massive deviation saddle with a support structure to resist large transversal forces from the deviation of the cable under tension. Most of the time, the anchorage region is placed on a large foundation built in the ground.
Some suspension bridges are of the “self-anchored” type, which means that the main suspension cables are, at one or both of their ends, anchored by means of an anchoring system mounted on the bridge deck.
In such a case, the forces exerted by the suspension cable are taken up by the compression of the deck and/or by piers built underneath and connected to the deck by tie-down members. In such an application, the bulk of the anchorage systems for the suspension cables is very problematic, so that it may be impossible to install them on the deck.
To alleviate these difficulties, it may be considered to replace a pair of suspension cables by only one cable forming a loop below the deck in the region where it connects with the deck. However, such a loop arrangement generates other problems. In particular, it is extremely difficult, if feasible, to put in place thousands of individual wires parallel to each other along a path of several hundreds of meters extending alternately above and below the deck. In addition, assuming that the latter difficulty is overcome, very large friction forces are induced in the curvature region where the cable loops under and around the deck to sustain it. Such friction occurs as the load is applied on the suspension cable, i.e. as the hangers are attached and tensioned. It may result in damage to the cable and/or to the deck. Trying to avoid such damage requires an additional tensioning system on the lower face of the deck to equalize the traction forces undergone by the cable below and above the deck, which further complicates the structure and its construction.
In view of these problems, an object of the present invention is to provide a method making it possible to provide a relatively compact anchorage for a cable consisting of multiple wires in a parallel bundle arrangement.